Estimating sailing times

[dmksails]

I've a question concerning the best way to estimate your total sailing time based not only on distance, but also considering wind speed and direction. (I'll ignore current for now)

For instance, My destination is 20 miles away, due north. Wind is also out of the north, so I'm not going to be sailing a straight course, but tacking to work my way north. Assume the average speed I'll make is 4 knots. How long will it take me to get to my destination and how did you determine this?

Thanks!

[carstenb]

Originally Posted by dmksails

I've a question concerning the best way to estimate your total sailing time based not only on distance, but also considering wind speed and direction. (I'll ignore current for now)

For instance, My destination is 20 miles away, due north. Wind is also out of the north, so I'm not going to be sailing a straight course, but tacking to work my way north. Assume the average speed I'll make is 4 knots. How long will it take me to get to my destination and how did you determine this?

Thanks!

This will depend a lot on your boat - how close to the wind can you sail? If you figure 45 degrees to true wind and perfect 90 degree tacks, you'll sail something like 40nm or 8 eight hours- but all that is pure theory, because it also assumes the wind is exactly constant and coming from the exact same direction the entire time - not gonna happen.

If you can't tack perfect 90 degrees, you'll sail more, The length of your optimal tack is also called velocity made good - some wind instruments can figure this out for you, otherwise You tube it - it will tell how to.

[SVHarmonie]

Originally Posted by dmksails

I've a question concerning the best way to estimate your total sailing time based not only on distance, but also considering wind speed and direction. (I'll ignore current for now)

For instance, My destination is 20 miles away, due north. Wind is also out of the north, so I'm not going to be sailing a straight course, but tacking to work my way north. Assume the average speed I'll make is 4 knots. How long will it take me to get to my destination and how did you determine this?

Thanks!

There are two numbers you need. Your angle to the TRUE wind, and your leeway.

For most monohull cruising boats, the best angle to sail relative to the true wind is about 50 to 52 degrees, maybe a bit better, maybe a bit worse. Leeway can be a bit trickier to come by, but if you look at your instruments and compare your heading to your course over ground (in the absence of current!) you'll come very close. Typically, for a deep draft cruising boat sailing in moderate winds, and calm seas, 5 degrees would be very optimistic, and 10 degrees very conservative. So let's call it 7 degrees.

So we are actually moving at an angle of ABOUT 58 degrees to the true wind. at 4 knots. With a little trigonometry we see that our speed straight upwind is equal to the cos(58 deg) * 4 knots. or (0.53 * 4) = 2.2 knots.

So your 20 mile trip straight upwind will take you 20/2.2=9.1 hours. It makes no real difference how many times you tack, the distance is the same.

Different boats will take different times. Catamarans typically can't sail as close to the wind, and make more leeway, but also go a bit faster.

Now, I promise, we are going to see a long list of people say "My boat does a LOT better than that!" and a few of them will be right. If you are sailing a FARR40 you might be sailing at 40 or even 37 degrees to the true wind, but that's not a typical cruising boat...

[StuM]

Others have given you VMG solutions. The other way to look at it is to look at the isosceles triangle(s) you sail. For every 1 mile towards the destination, you have to sail out on one leg and back the same distance.

Basic trigonometry tells us that for every mile made good you have to sail the following distances based on your track angle (true wind angle plus leeway)

1.3 miles @ 40°
1.4 miles @ 45°
1.55 miles @ 50°
1.74 miles @ 55°
2 miles @ 60°

So if your boat's track makes 50° either side of your desired heading, you will cover 1.55 x 20 = 31 miles at 4 knots = 7 3/4 hours.

[Tillsbury]

Don’t forget tide or current, as with these sorts of situations (VMG in the region of 2kt) it makes a huge difference. It about 5nm from my home marina to the heads of the main harbour. On a nice sailing day, the wind is very often straight down the harbour. Even though our tides are minimal compared with much of the world, the difference between and incoming and outgoing tide can be the difference between it taking 1 and 2 hours to make the entrance. Coming back home downwind, it takes somewhere between 20 and 30 minutes depending on the tide state. The difference when you’re travelling upwind is a much bigger issue.